In communication systems there is often a need to shift a signal from one frequency band to a different frequency band, with very little or preferably no distortion occurring. This frequency shifting is done by a device or circuit called a mixer. The mixer typically generates the signal in a new frequency band (often called the Intermediate Frequency signal or IF signal when the mixer is used in a radio receiver) from the original signal (often called the Radio Frequency signal or RF signal when the mixer is used in a radio receiver) using an unmodulated signal (often called the Local Oscillator frequency or LO when the mixer is used in a radio receiver).
Mixers come in several forms, but they typically rely on well known non-linearities to achieve the desired mixing. Unfortunately, those non-linearities can also give rise to undesirable intermodulation distortion. A lot of the technical literature with respect to mixers discuss about how to take advantage of the non-linearities to achieve the desired mixing while reducing the unwanted intermodulation distortion.
In a typical mixer, a down-converted Intermediate Frequency (“IF”) signal is derived which is a nonlinear harmonic product of Radio Frequency (“RF”) and Local Oscillator (“LO”) signals. A typical mixer uses a device's non-linearity to achieve the mixing. Because of this, achieving highly linear mixer is challenging, especially for a large third order input intercept point (“IIP3”).
IIP3 is a figure of merit used in describing the linearity of many circuits, including mixers. IIP3 is determined by finding the intersection of a plot of the output power versus the input power for a first order term of the Taylor series expansion of the circuit transfer function and a plot of the third order term of the series expansion for the circuit transfer function. The third order distortion products are undesirable, so the bigger the ratio, the better (less distorted) the circuit being analyzed. So a higher IIP3 is better than a lower IIP3.
Mixers can also have a conversion loss which is defined as the ratio of the power of the signal in the new frequency band (typically the signal at the IF frequency) to the power of the original signal in the original frequency band (typically the signal at the RF frequency). If the mixer has a built in amplifier, then it can exhibit a conversion gain.
Mixers are often called resistive mixers when transistors are operated under turned-off DC bias condition to achieve the desired frequency mixing. When one or more Field Effect Transistors (FETs) are utilized in a resistive mixer, it is the non-linearities in the channel conductance of the one or more FETs which are utilized to achieve frequency mixing.
To reduce and/or minimize conversion loss and maximize IIP3 (to lower third order distortion products), the mixer must be very linear during the intermodulation of RF frequencies and sufficiently non-linear during the intermodulation of RF and LO frequencies to achieve the desired mixing of the RF and LO frequencies.
A typical balanced (or double balanced) resistive microwave mixer uses one device at one stage and usually contains matching components (such as micro-strip line or capacitors) to try to obtain the best conversion loss with reasonable linearity.
Typically, when a linear resistive mixer is designed, each unit cell has one transistor and its size and gate bias are adjusted to try to obtain the best linearity performance along with matching condition. However, since there is trade-off relationship between IIP3 and conversion loss in this approach, better IIP3 cannot typically be achieved without sacrificing conversion loss.
In some cases, selective drain bias can be applied to utilize device output conductance as described in J. A. Garcia, J. C. Pedro, M. L. De la Fuente, N. B. Carvalho, A. Mediavilla, and Tazon, A., “Resistive FET mixer conversion loss and IMD optimization by selective drain bias.” Microwave Symposium Digest, IEEE MTT-S International Volume: 2 (1999) and in J. Kim and Y. Kwon, “Intermodulation analysis of dual-gate FET mixers,” Microwave Theory and Techniques, IEEE Transactions on Volume: 50, Issue: 6 (2007). However, their resulting IIP3 may still be limited and a more complex layout may be required.